There's a handful of utility libraries that spring to mind quickly; glib provides a wide variety of useful little utilities, including:
doubly- and singly-linked lists, hash tables, dynamic strings and string utilities, such as a lexical scanner, string chunks (groups of strings), dynamic arrays, balanced binary trees, N-ary trees
(And yes, glib is useful even in non-graphical environments; don't let its GNOME-background fool you. :)
The Apache portable runtime is a library that helps abstract away platform-specific knowledge; I've seen a handful of programs use it. It feels like enough programmers are content with "It runs on Linux" to not really worry about platform differences, and forgo learning Yet Another Library as a result. It feels more like a systems-level toolkit:
Memory allocation and memory pool functionality, Atomic operations, Dynamic library handling, File I/O, Command argument parsing, Locking, Hash tables and arrays, Mmap functionality, Network sockets and protocols, Thread, process and mutex functionality, Shared memory functionality, Time routines, User and group ID services